Public telephone networks currently utilize a combination of land lines, microwave repeaters, undersea cables and satellites operating in geosynchronous orbits. In the past decade, phone service has been enhanced by cellular systems. In response to burgeoning global demand for telephone service, communications utilities continue to seek the most effective means of offering more capacity at the lowest cost. The cost of supplying additional capacity by adding more terrestrial facilities is quite high. As an alternative, greater levels of communications services can be achieved employing large satellites that operate in geosynchronous orbit. This approach is also very expensive, and is unsuitable for providing direct service to phone customers using portable and mobile terminals because of the extremely high power levels that would be required to communicate with satellites flying at an altitude of 22,300 miles. In an effort to overcome the limitations of geosynchronous systems, several companies have recently proposed various networks of low Earth orbit satellites. These networks, however, would introduce their own set of formidable problems, because of the rapid motion of satellites over the regions on the Earth that they are designed to serve.
Several publications noted below disclose various systems that pertain to communication systems that are designed to operate on the Earth's surface or in conjunction with satellites flying in geosynchronous and low Earth orbits.
In U.S. Pat. No. 4,931,802, Assal et al. disclose a satellite which is designed exclusively for operation in a geosynchronous orbit. Assal et al. describe a multi-beam communication satellite, which the inventors say exploits the advantages of using a large number of small pencil beams to reduce satellite and terminal power requirements and to increase frequency reuse. The specification of this reference describes various implementations of a satellite-switched time division multiple access (TDMA) scheme. Assal et al. clearly intended to use their invention only with satellites whose positions remain fixed with respect to the Earth. They did not anticipate low Earth orbit satellites which do not fly in geostationary orbits, and did not address the problems faced by a low Earth orbit communication system which operates far below geostationary altitudes. Nothing in the Assal et al. specification describes a beam steering scheme that might be used to compensate for the rapid motion of the satellite with respect to Earth, changes in satellite attitude, or the Earth's rotation. Nor does the specification mention a means for handling the "hand-off" of a terminal from one satellite beam to another beam or to another satellite which would be required in a low Earth orbit system.
In U.S. Pat. No. 5,107,925, Bertiger et al. disclose a multiple beam space antenna system for facilitating communications between a satellite switch and a plurality of Earth-based stations.
Antoine Roederer describes a feed device for multibeam antennas in U.S. Pat. No. 5,115,248.
Samuel Gubin discloses a communication system employing a satellite capable of providing full communication coverage of an irregularly shaped area on the surface of the Earth in his U.S. Pat. No. 3,541,553.
Bertiger, Leopold and Peterson describe a "Satellite Cellular Telephone and Data Communication System" in European Patent Application No. 891 184 58.2. This application sets out some of the details of Motorola's proposed "Iridium.TM." communication system. The Iridium.TM. system is currently designed to utilize sixty-six (66) satellites in low Earth orbit which would generate relatively large footprints of radio beams due to their extremely low mask angle of eight and one half degrees (81/2.degree.). Because of these very large footprints, the communications capacity that may be offered by the Motorola network would be substantially constrained. In addition, this system would employ "satellite-fixed cells" which are not defined by any constant boundaries on the Earth. These cells would sweep over vast regions of the Earth at very high speeds as the Iridium.TM. satellites fly overhead. This method of using satellite-fixed cells introduces extremely complicated "hand-off" problems when one satellite moves out of range of supplying service with a subscriber. At that time, another satellite must assume the responsibility of supporting the subscriber's call without interruption.
In an article entitled "A Multibeam Active Antenna for an European Contiguous Coverage at Ku-Band" published in the Proceedings of the IEEE, 1989, Bartolucci et al. discuss active multibeam antennas for European coverage at Ku-bands.
Acampora et al. explain their "Metropolitan Area Radio System Using Scanning Pencil Beams" in the 1991 edition of the Proceedings of the IEEE. The authors of this paper propose a metropolitan area radio system that would provide continuous 360 degree coverage over a large service region from a centrally located base station. The authors claim that this approach differs from others in that the base station would blanket the service region with a raster of very narrow pencil beams which could be rapidly scanned to any position in synchronism with the switching sequences of a TDMA assignment.
In the article entitled "Advanced Communications Technology Satellite (ACTS) and Potential System Applications" published in the July, 1990 edition of the Proceedings of the IEEE, Wright et al. describe an Advanced Communications Technology Satellite System and potential satellite communications scenarios.
Direct world-wide telephone services via satellite that are currently available to persons using portable, mobile and fixed terminals are extremely limited and too expensive for use by all but a few. The problem of providing an economically viable satellite network for voice, data, and video which can be used by subscribers around the globe has presented a major challenge to the communications business. The development of a high power satellite system which can transmit and receive radio signals to portable, mobile, and fixed terminals on the land and sea and in the air without the intermediate steps of routing traffic through land-based equipment would constitute a major technological advance and would satisfy a long felt need within the electronics and telephone industries.